Transcript Slide 1

A Very Low Bit Rate
Protection Layer to Increase
the Robustness of the AMRWB+ Codec against Bit Errors
Philippe Gournay
Université de Sherbrooke
Département de Génie Electrique et Informatique
Sherbrooke, Québec, Canada
Outline
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Context
The AMR-WB+ Codec
Bit Sensitivity Study
The Protection Layer
Evaluation Results
Audio Demonstration
Conclusion
Context
• Challenging Applications for AMR-WB+
– Streaming and Multimedia Messaging in GSM,
DVB-H Mobile TV, Digital Radio
• Channel Impairments
– Lost Packets
• Concealment Procedure
– Bit Errors
• Channel Coding (Forward Error Correction)
• Inherent Robustness of the codec
• Additional Protection Layer
Overview of the AMR-WB+ codec
• Speech and Music
• Mono and Stereo Inputs
• Wide Range of Audio Bandwidth
– From 8 to 48 kHz
• Wide Range of Bit Rate
– 6 to 36 kbps in mono, 7 to 48 kbps in stereo
• Hybrid Coding Model
– Time (ACELP) and Frequency (TCX) Domain
ACELP
1 frame
Audio
Mode
Selection
TCX
1, 2 or 4 frames
Mode Index, ISF
PACKETIZATION
The AMR-WB+ Encoder
Bitstream
AMR-WB+ Frame Structure
(a)
ACELP
ACELP
(b)
Short TCX
ACELP
(c)
ACELP
ACELP
Medium TCX
Long TCX
One super-frame = 80 ms @ 12.8 kHz
• Three out of the 26 possible ACELP/TCX coding
configurations
AMR-WB+ Embedded Bitstream
Structure
Core Mode
Core
Bitstream
(ACELP or
TCX)
Stereo Mode
BWE
(mono/right)
BWE
(left)
Core codec
Low-band
Mid-band
Stereo extension
• For medium and long
TCX, the packetization
procedure is responsible
for distributing the
bitstream among the
required number of
packets.
Bit Sensitivity Study
• Determine Individual Bit Sensitivities
– Prior Knowledge (ACELP, AMR-WB Standard)
– SNR Measurements
• Bit Sensitivity Classes
– Class A: Highly sensitive
– Class B: Moderately Sensitive
– Class C: Not Sensitive
• Useful for U.E.P. (Unequal Error Protection)
SNR (dB)
Segmental SNR (8.67 kbps mono)
100
100
80
80
60
60
40
40
20
20
0
100
SNR (dB)
ACELP
50
100
150
200
Bit position
0
50
100
150
200
100
80
80
60
TCX40
60
40
40
20
20
0
TCX20
100
200
300
Bit position
400
0
TCX80
200
400
600
800
The Protection Layer
• Additional Protection Layer
– Standard AMR-WB+ Payload Kept Unchanged
• Very Low Bit Rate
– 16 bits per frame (480 bits @ 32 kbps stereo)
• Performs Error Detection and/or Correction
– Class A: Error Detection and Correction
– Class B: Error Detection
– Class C: Nothing
• Depends on the Core and Stereo Modes
Core Protection Layer (1)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
Medium TCX
Packet 1
Packet 2
ACELP
Short TCX
Mode redundancy
Mode redundancy
Mode redundancy
Mean energy
1st 2nd ISP subvect.
Pitch SF1
Gain SF1
Pitch SF2
Gain SF2
Pitch SF3
Gain SF3
Pitch SF4
Gain SF4
Global gain
Global gain
1st ISP subvector
3rd ISP subvector
1st ISP subvector
2nd ISP subvector
Mode redundancy
3rd ISP subvector
First 8 bits of AVQ
codebook numbers
unused
unused
Core Protection Layer (2)
Long TCX
0
1
2
3
4
5
6
7
8
9
10
11
12
13
Packet 1
Packet 2
Packet 3
Packet 4
Mode redundancy
Mode redundancy
Mode redundancy
Mode redundancy
3rd ISP subvector
First 8 bits of AVQ First 8 bits of AVQ
codebook numbers codebook numbers
Global gain
1st ISP subvector
First 8 bits of AVQ Next 8 bits of AVQ Next 8 bits of AVQ
codebook numbers codebook numbers codebook numbers
2nd ISP subvector
unused
unused
unused
unused
unused
unused
Stereo Extension Protection Layer
14
15
Mode 0
Mode 1
Mode ctrl.
Bal. & gain
Mode ctrl.
Bal. & gain
Mode 2
Mode 2
Packet 1
Mode ctrl.
Balance
Packet 2
Mode ctrl.
Gain
Stereo mode 3
14
15
Packet 1
Mode ctrl.
Balance
Packet 2
Mode ctrl.
unused
Packet 3
Mode ctrl.
Gain
Packet 4
Mode ctrl.
unused
Subjective Evaluation
(32 kbps stereo)
5
4
3
2
1
Note: bit errors are also applied to the protection layer.
Audio Demonstration
• AMR-WB+ @ 32 kbps stereo
– Mode Index=40, ISF=1.333
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Original File
Clear Channel
Standard Codec, 0.5% BER
Modified Codec, 0.5% BER
Audio Demonstration
• AMR-WB+ @ 32 kbps stereo
– Mode Index=40, ISF=1.333
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Original File
Clear Channel
Standard Codec, 0.5% BER
Modified Codec, 0.5% BER
Audio Demonstration
• AMR-WB+ @ 32 kbps stereo
– Mode Index=40, ISF=1.333
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•
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•
Original File
Clear Channel
Standard Codec, 0.5% BER
Modified Codec, 0.5% BER
Audio Demonstration
• AMR-WB+ @ 32 kbps stereo
– Mode Index=40, ISF=1.333
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•
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•
Original File
Clear Channel
Standard Codec, 0.5% BER
Modified Codec, 0.5% BER
Conclusion
• AMR-WB+ Bit Sensitivity Study
– Sensitivity Classes A, B and C
– Useful for U.E.P.
• Additional Very Low Bit Rate Protection Layer
– 16 bits per frame
– Error Detection and Correction
– Enhance Robustness against Bit Errors
• Significant Subjective Quality Improvement
– Decoded Audio is Free of Major Artifacts at a
Significant 0.5% BER